Method for producing pieces having a modified surface

ABSTRACT

The invention relates to a method for modifying piece surfaces consisting in bringing pieces into contact with at least one type of a modifying agent in such a way that the modification of the surface is carried out.

The present invention relates to processes for producingsurface-modified workpieces made from metal and/or one or more alloys,to the workpieces produced by means of these processes and to processesfor joining at least two such workpieces.

BACKGROUND

The direct coating of aluminum or aluminum alloys by means of organiccoating systems is virtually impossible on account of the inadequatebonding of the organic coating systems to the aluminum. Therefore, toimprove the bonding between the starting material and organic coatingsystems, it is known to subject aluminum or aluminum alloys to what isknown as a boehmite treatment, in which hot water or hot steam, ifappropriate together with ammonia or amines, is brought into contactwith the workpiece, so as to form or thicken an aluminum oxide orboehmite layer. This then allows an organic coating to be applied. EP 1142 663 A1 describes boehmite processes, in which deionized water attemperatures of approximately 100° C. or steam at temperatures of 150°C. is used to surface-modify aluminum parts. U.S. Pat. No. 3,945,899 hasdisclosed a boehmite treatment of aluminum parts with water at atemperature of from 65 to 100° C. or steam at temperatures of 100 to180° C., with the addition of amines and ammonia further boosting thealuminum oxide layer.

It is also known from this document to subject aluminum or its alloys toa chemical surface treatment using aqueous solutions of chromates orphosphates, in order in this way firstly to increase the bonding andsecondly to reduce the susceptibility to corrosion. This conversiontreatment, as it is known, is also known from Stolzenfels(Industrie-Lackierbetrieb, [Industrial coating operation] No. 3, pp.93-98, Curt R. Vincentz Verlag), which describes chromating treatmentsof aluminum workpieces at temperatures of from 20 to 50° C. Riese-Meyeret al. (Aluminium 1991, No. 12, pages 1215-1221) describes chemicalconversion treatments by means of layer-forming phosphating steps andchromating steps, allowing the coating bonding and corrosion resistanceof aluminum workpieces to be improved. According to this document too,the chromating is carried out at a temperature of from 20 to 30° C. or30 to 40° C.

SUMMARY OF THE PREFERRED EMBODIMENTS

However, the abovementioned processes have proven disadvantageous interalia on account of energy considerations. Since the workpieces which areto be modified in accordance with the prior art are generally at roomtemperature, i.e. are used in a cooled state with respect to a precedingbrazing or joining process, considerable handling and time are requiredfor the surface modification of such workpieces. Moreover, the shapedbodies made from aluminum or aluminum alloys which are to be modified inthe prior art tend toward what is known as coarse-grain formation, onaccount of the slow cooling process carried out after they have beenproduced.

Therefore, the present invention is based on the technical problem ofproviding processes which overcome the abovementioned drawbacks, inparticular of providing processes which provide workpieces made fromaluminum or aluminum alloys with improved bonding and/or corrosionresistance at lower cost, in particular workpieces whose structure isdistinguished by reduced coarse-grain formation compared toconventionally produced aluminum or aluminum alloy workpieces whilehaving a corrosion resistance and bonding which are as good if notbetter.

The present invention solves the technical problem on which it is basedby providing a process for producing surface-modified workpieces madefrom aluminum or one or more aluminum alloys, comprising the steps ofproviding the workpiece which is to be modified and is at a temperatureof from 40 to 700° C., preferably 80 to 550°, in particular 300 to 550°C., and then treating this workpiece at the abovementioned temperaturewith at least on modifying agent to obtain the surface-modifiedworkpiece, the at least one modifying agent being at a temperature offrom −200° C. to 100° C., preferably at most 80° C. and in particular atleast 0° C., preferably 20 to 65° C. Therefore, the invention solves theproblem on which it is based in particular by providing, at a relativelyhigh temperature, a workpiece whose surface is to be modified andbringing it into contact with at least one modifying agent which is at alower temperature, so as produce a surface-modified workpiece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the context of the present invention, a workpiece is to be understoodas meaning an object of any desired form which, by way of example, maybe in the form of a shaped body, i.e. a body of defined form, oralternatively also in the form of granules or a powder.

In a preferred embodiment, the workpiece is in the form of a heatexchanger or a significant component thereof. In a preferred embodimentof the present invention, the workpiece which is to be modified iscomposed of aluminum, magnesium, copper or one or more aluminum and/ormagnesium and/or copper alloys, i.e. it consists of aluminum or one ormore alloys or it substantially comprises aluminum or one or morealuminum alloys, for example, in amounts of at least 50, 60, 70, 80, 90,95 and in particular 99% by weight, based on the weight of theworkpiece.

In its first configuration, the invention provides for the workpiecewhose surface is to be modified to be provided in heated form, theworkpiece to be modified being at a temperature of from 40 to 700° C.,in particular 100 to 550° C., preferably 150 to 550° and particularlypreferably 300 to 550° C. In a preferred embodiment, this can beachieved by virtue of the fact that the workpiece is used according tothe invention when it is still hot immediately following its productionprocess, e.g. after it emerges from a brazing zone, after thermaljoining processes or after heating in chamber furnaces, so as to exploitthe heat capacity which is present in these workpieces. The workpiecewhich is at one of the abovementioned temperatures, for example atemperature of from 80 to 550, in particular 300 to 550° C., isconsequently treated by utilizing its existing elevated temperature,i.e. by being brought into contact with at least one surface-modifyingagent, in such a way as to achieve the desired surface modification ofthe workpiece.

The surface modification which is desired in accordance with theinvention is in this case a chemical conversion, i.e. the formation of aconversion layer and/or the formation or thickening of a boehmite layeror aluminum oxide layer. In a preferred embodiment, the surfacemodification which is desired in accordance with the invention may alsocomprise the incorporation of corrosion-inhibiting substances into thealuminum oxide or boehmite layer and/or the production of conversionlayers by chemical-physical modification of possibly existing fluxlayers.

For example, it is possible to provide for workpieces which alreadyinclude a CAB flux layer on account of a preceding CAB brazing processto be treated by means of the procedure according to the invention insuch a way that the existing CAB flux layer is chemically-physicallymodified. The procedure according to the invention may result in theexisting flux layer being doped, for example with metals from maingroups I, II, III or IV or from the transition groups, in particular IVto VI, and/or having its oxygen content increased. The treatmentaccording to the invention results, inter alia, in an improvedresistance to corrosion.

The use of a modifying agent at a low temperature, in particular atemperature from −200° C. to 100° C., preferably 0 to 80° C.,particularly preferably 20 to 65° C., which is preferred in accordancewith the invention leads, when treating a workpiece at a hightemperature of from 80 to 700° C., in particular 300 to 550° C., torapid cooling of the workpiece and therefore advantageously toprevention or reduction of the coarse-grain formation in the workpiece.Consequently, the resulting workpieces under certain circumstancesadvantageously differ in terms of their structure from conventionallyproduced workpieces.

The treatment of flux (CAB)-coated workpieces which is preferablyprovided in accordance with the invention under certain circumstancesleads to an advantageous scaly, continuous and rounded appearance of theflux layer of the aluminum part, which differs from the open-pore,sharp-edged and platelet-like appearance of untreated flux-coatedaluminum parts.

In a further configuration of the present invention, the presenttechnical problem is solved by providing a process for producingsurface-modified workpieces made from a metal and/or one or more alloys,comprising the steps of providing the workpiece that is to be modifiedand treating this workpiece with at least one modifying agent to obtaina surface-modified workpiece, the at least one modifying agent being ata temperature of from 80 to 550° C., preferably 100 to 200° C.,preferably 150 to 200° C.

In this configuration of the invention, the workpiece which is to bemodified may be at a temperature of from 15 to 80° C., in particular 40to 65° C., but also 20 to 60° C., in particular 20 to 40° C., preferably20 to 30° C., or from 80 to-700° C., preferably 100 to 700° C.,preferably 150 to 700° C., in particular 300 to 550° C. Thisconfiguration of the present invention also allows the advantageoussurface-modification of a workpiece, in particular the production of ametal oxide or boehmite layer, the thickening of an existing metal oxideor boehmite layer, the incorporation of corrosion-inhibiting substancesinto a metal oxide or boehmite layer and/or the production of conversionlayers by chemical-physical modification, in particular of flux layers.

Following the surface modification of the workpiece, the workpieces canbe treated further in the usual way, in particular by being rinsed anddried. Of course, it is also possible to carry out a further coatingoperation, for example by means of organic coating systems. The presentprocess therefore provides part of the production process of aworkpiece, for example a heat exchanger. The procedure which is providedfor in accordance with the invention, within this production process,leads to a reduction in the production costs for workpieces, to a savingon energy and resources, in particular by utilizing existing heatcapacities of the workpieces, and to the reduced use or the avoidance ofthe use of aggressive chemicals for the surface treatment.

Under certain circumstances, all known chemical elements, compounds,mixtures or other compositions may be suitable for use as the modifyingagent. A modifying agent which is preferably used is one or morecompounds, in particular one or more metal salts of one or more elementsfrom the transition groups of the PSE, in particular transition groupsIV to VI of the PSE (periodic system of the elements), for exampletitanium, hafnium, vanadium, tantalum, molybdenum, tungsten and inparticular zirconium.

In a further embodiment of the present invention, the modifying agentmay be one or more compounds, in particular one or more metal salts ofone or more elements from main groups I, II, III, and/or IV of theperiodic system of the elements, for example a metal salt of beryllium,barium, in particular of magnesium or calcium or sodium or potassium.

In a further embodiment of the present invention, the modifying agentmay be one or more compounds of one or more elements from main groups V,VI, VII and/or VIII of the periodic system of the elements. In apreferred embodiment of the invention, the abovementioned metals may bein salt form with anions selected from the group consisting ofchlorides, carbonates, in particular hydrogen carbonates, nitrates,sulfates, peroxides and phosphates. In particular the metal salts ofelements from main groups I and II, for example potassium, sodium andcalcium, may be in the form of a hydroxide solution, i.e. KOH, NaOH orCa(OH)₂, or in the form of a borate, aluminate, silicate or halide, inparticular fluoride.

In a further preferred embodiment of the invention, the at least onemodifying agent is a CAB-flux (“controlled atmosphere brazing”) of thegeneral formula K_(x)AlF_(y) where x equals 1 to 3 and y=4 to 6, forexample potassium aluminum hexafluoride and/or Cs_(x)AlF_(y).

In a further preferred embodiment, the modifying agent used is anammonium salt, such as for example ammonium fluoride or ammoniumcarbonate, potassium fluoride, sodium or potassium silicate, sodium orpotassium borate, sodium or potassium aluminate, crosslinkable, inparticular organometal, such as for example organozirconium ororganotitanium, or organosilicon compounds, or alternatively hydrogenperoxide.

In a particularly preferred embodiment, the CAB flux, ammonium saltand/or potassium fluoride are used in alkaline phase, in particular inthe form of aqueous, preferably alkaline solutions or alkaline vapors oraerosols, to treat the workpiece.

The metal compounds of one of the elements of the transition groups, inparticular transition groups IV to VI, or main groups I, II, III or IV,may be in organic and/or inorganic phase, preferably in aqueous phase,in particular in liquid or gaseous phase, preferably in aerosol form oras a vapor. The water which is used for the dissolution is preferablydeionized water.

In a further preferred embodiment, there is provision for water,preferably deionized and distilled water, to be used as modifying agentfor treating the surface of the workpiece. Of course, according to theinvention it is also possible for the surface-modifying agent used to beaqueous solutions of ammonia, of amines, in particular primary,secondary or tertiary amines, for example mono-, di- ortriethanolamines, dimethylethanolamines, organic acids or salts ofammonia, amines, halogenated organic compounds and/or organic acids. Ofcourse, it is also possible to use mixtures of the abovementionedmodifying agents.

It is preferable to use a solution of 0.1-1% of KOH and/or 0.1-1% ofNH₄OH and/or 0.1-1% of K_(x)AlF_(y) (x=1 to 3, y=4 to 6) and/or 0.1-1%of Ca(NO₃)₂ and/or 0.1-1% of salts of the elements of transition groupsIV to VI of the PSE in deionized water.

In a particularly preferred embodiment of the present invention, thestarting workpiece used for the process according to the invention is aCAB flux-coated workpiece which results from a CAB brazing process andis treated, under the conditions mentioned above, with one or more ofthe modifying agents used. In particular if the surface is treated withwater or aqueous solutions, it is possible to obtain a modified surfacewith an increased oxygen content; depending on the type of modifyingagent used, this surface may also be doped, for example with one or moreof the metals from main group I, II, III or IV or from the transitiongroups, in particular transition groups IV to VI, or other modifyingagents used.

In another preferred embodiment, the invention provides for the metalsalt, the CAB flux, ammonium salt and/or potassium fluoride or anotherconstituent of the modifying agent to be used in a matrix, for example amatrix of organic and/or inorganic solvents or mixtures thereof, totreat the surface of the workpiece. In this case, the matrix containsorganometal, in particular organosilicon compounds. In particular, thematrix contains organic and/or inorganic polymers, or alternatively amixture of the abovementioned substances.

In a particularly preferred embodiment, there is provision for the metalsalt, the CAB flux, ammonium fluoride and/or potassium fluoride oranother constituent of the modifying agent to be used in a concentrationof from 10 ppm to 100000 ppm, preferably from 50 ppm to 10 000 ppm inthe treatment.

The at least one modifying agent is preferably brought into contact withthe workpiece that is to be modified by the workpiece being immersed inthe at least one agent and impregnated with the agent or by it beingrinsed or flooded with the at least one modifying agent and thusimpregnated with it, or by the at least one modifying agent beingsprayed onto the workpiece, in particular by means of what is known asairless or ultrasound atomization, or being brought into contact with itis some other form.

In a particular embodiment, it is possible to provide for the modifyingagent to act on the workpiece at a pressure which is higher thanatmospheric pressure. In the case of spraying, in addition to compressedair it is also possible to use other gases, for example oxygen,nitrogen, fluorine, ozone or steam.

Examples of metal salts used include aqueous solutions of Ca(NO₃)₂ orZr(NO₃)₄, in particular in concentrations of between 0.1% and 5%, the pHof which is preferably between 5.5 and 7.5 to 8. The temperature of useis in this case advantageously between 40° C. and 60° C. Under certaincircumstances, it may also be advantageous to add from 0.005% to 5% oftetraethylammonium tetrafluoroborate.

A solution of this type is used in particular to treat a brazed,preferably CAB-brazed, heat exchanger.

According to an advantageous embodiment, the at least one modifyingagent includes a biocidal agent. By way of example, it is desirable fora heat exchanger used in a heating and/or air-conditioning installationto have a germ-inhibiting action which is produced as a result.

According to a further advantageous embodiment, the modifying agentproduces an oxidation-inhibiting action by virtue of the agent includingan oxidation inhibitor or producing an oxidation inhibitor on thesurface by means of the process according to the invention. An oxidationinhibitor is provided, for example, by a metal oxide or boehmite layer.

Of course, the invention also relates to surface-modified workpiecesproduced by means of the abovementioned processes, in particularsurface-modified heat exchangers made from aluminum or aluminum alloys.

In an advantageous refinement of the invention, in a further step thesurface-modified workpiece is provided with one or more organic orinorganic coating systems which particularly advantageously hasgerm-inhibiting and/or hydrophilic or hydrophobic properties. It ispossible for these coating layers to be applied with or without anintervening drying step.

The invention is also implemented by a process for joining two or moreworkpieces. A production process which includes a process of this typecan be carried out quickly, so that it is possible to reduce themanufacturing costs. This applies in particular to processes forcohesively joining a plurality of workpieces, but is also achieved inother joining processes, such as for example mechanical joining, inwhich case under certain circumstances it may be necessary to (re)heatthe workpiece in order to apply the process according to the invention.

The invention is also achieved by an apparatus for cohesively joining atleast two workpieces, in which a temperature-control chamber forcontrolling the temperature of, in particular heating, the workpieces,has a device for applying a surface-modifying agent to at least one ofthe workpieces. The temperature-control chamber advantageously servessimultaneously for cohesive bonding, such as for example brazing orsoldering or welding, and surface-modifying the workpieces.

The device for applying a surface-modifying agent is preferably designedas a spray nozzle, the temperature of which can itself particularlypreferably be controlled in order for the process according to theinvention to be carried out. It is equally possible for the temperatureof the surface-modifying agent to be controlled in a feedline of thedevice.

Otherwise, the apparatus according to the invention may under certaincircumstances be constructed in a known way, for example as a vacuum orcontinuous brazing furnace.

Further advantageous configurations of the present invention will emergefrom the subclaims.

1. A process for producing surface-modified work pieces made from atleast one of a metal and one or more alloys, comprising the steps of:providing the workpiece which is to be modified, wherein the workpieceto be modified is provided at a temperature of from 300 to 550° C., andafter providing the workpiece at the temperature of from 300 to 550° C.,treating the workpiece with at least one modifying agent to obtain thesurface-modified workpiece, wherein the at least one modifying agent isat a temperature of at least 0° C. to at most 100° C., wherein the atleast one modifying agent comprises a metal salt of an element from oneof transition groups I to VI of the periodic table of elements and acompound of an element from groups V, VI, VII or VIII of the periodictable of elements, wherein the at least one modifying agent comprises acontrolled atmosphere brazing (CAB) flux, wherein the metal salt is inan aqueous phase, the pH of which is between 3 and 10, wherein thetreating step comprises contacting the workpiece with at least onemodifying agent in aerosol and/or vapor form, and wherein the metal andone or more alloys is at least one of aluminum, magnesium, copper, analuminum-based, a magnesium-based, and a copper-based alloy.
 2. Theprocess as claimed in claim 1, wherein the modifying agent comprisespotassium aluminum hexafluoride.
 3. The process as claimed in claim 1,wherein the modifying agent is at least one of ammonium fluoride,potassium fluoride, sodium or potassium silicate, sodium or potassiumborate, sodium or potassium aluminate, at least one crosslinkablecompound, and an organometal.
 4. The process as claimed in claim 1,wherein the aqueous phase has a pH between 4 and
 8. 5. The process asclaimed in claim 1, wherein the CAB flux is in a phase having analkaline pH.
 6. The process as claimed in claim 1, wherein the modifyingagent comprises deionized or distilled water, or an aqueous solutioncontaining ammonia, amines, gases or organic acids or their salts ormixtures thereof.
 7. The process as claimed in claim 1, wherein the stepof treating the workpiece comprises spraying the at least one modifyingagent the workpiece.
 8. The process as claimed in claim 1, wherein thestep of treating the workpiece comprises immersing the workpiece in theat least one modifying agent, which is in the form of aqueous solution,or by flooding the workpiece with the at least one modifying agent,which is in the form of the aqueous solution.
 9. The process as claimedin claim 1, wherein the at least one modifying agent is at least one ofa metal salt, a CAB flux, ammonium fluoride, potassium fluoride, sodiumor potassium silicate, sodium or potassium borate, sodium or potassiumaluminate, at least one of crosslinkable compound, and at least oneorganosilicon compound used in a matrix to treat the workpiece.
 10. Theprocess as claimed in claim 9, wherein the matrix comprises organic orinorganic solvents or mixtures thereof.
 11. The process as claimed inclaim 1, wherein at least one modifying agent is at least one of a metalsalt, a CAB flux, ammonium fluoride, potassium fluoride, sodium orpotassium silicate, sodium or potassium borate, sodium or potassiumaluminate, and an organometal used to treat the workpiece in aconcentration of from 10 ppm to 100000 ppm.
 12. The process as claimedin claim 1, wherein the workpiece to be modified is a heat exchanger.13. The process as claimed in claim 1, wherein the at least onemodifying agent includes a biocidal agent and/or a corrosion inhibitor,or the at least one modifying agent produces a biocidal agent and/or acorrosion inhibitor on a surface of the workpiece.
 14. A process forcohesively joining at least two workpieces, comprising the steps of: a)providing the workpieces, b) cohesively joining the workpieces to oneanother, and c) surface-modifying at least one of the workpieces,wherein step c) is carried out as described in the steps of claim
 1. 15.The process as claimed in claim 1, wherein the at least one modifyingagent is at a temperature of at most 80° C.
 16. The process as claimedin claim 9, wherein the crosslinkable compound comprises an organometal.17. The process as claimed in claim 16, wherein the organometalcomprises organozirconium or an organotitanium compound.
 18. The processas claimed in claim 11, wherein the metal salt, the CAB flux, ammoniumfluoride, potassium fluoride, sodium or potassium silicate, sodium orpotassium borate and/or sodium or potassium aluminate and/or organometalis/are used to treat the workpiece in a concentration of from 50 ppm to10000 ppm.
 19. The process as claimed in claim 12, wherein the heatexchanger is a CAB brazed heat exchanger.
 20. The process as claimed inclaim 3, wherein the organometal is at least one of an organozirconiumcompound, an organotitanium compound, and an organosilicon compound.